For the purpose of machining objects deep within the object material it is known to use ultra-short-pulse laser radiation with pulse durations within the femtosecond (fs) range (where appropriate, extending into the single-digit picosecond range), which is able to bring about a laser-induced optical breakthrough at the focus and, resulting from this, a photodisruption that is substantially restricted to the area of focus. The prerequisite for this is a transparency of the object being machined in respect of the laser radiation, which obtains, for example, in the case of machining operations on the human eye above a wavelength of about 300 nm. In the case of laser machining of the human eye, fs laser radiation is employed, in particular, for the purpose of generating incisions in the cornea or in other tissue parts of the eye, for instance within the scope of a LASIK treatment (LASIK: laser in-situ keratomileusis) for generating a flap, in the course of a corneal lenticle extraction for the purpose of generating a lenticular intracorneal lamella, or in the course of a corneal keratoplasty for the purpose of excising a piece of corneal tissue to be replaced or transplanted.
In all these forms of machining, a high positioning accuracy of the laser focus in all three spatial coordinates in the target tissue is required, in which connection currently conventional accuracy requirements stipulate a few μm and, in the best-possible case, permit positioning tolerances of only 1 μm or 2 μm.
At least the fs laser devices employed in laser surgery on the human eye often possess a mechanical interface unit, sometimes designated as a patient adapter, with a contact element that is transparent to the laser radiation and that exhibits a contact surface which has to be brought into planar abutting contact with the surface of the eye or generally with the object to be machined. The interface unit is, for example, an exchangeable module that can be coupled to focusing optics of the laser device. The contact element with its contact surface may serve as positional reference for the adjustment of the position of the radiation focus. Insofar as the eye is applied onto the contact element, a precise machining of the eye is possible, assuming a precise referencing of the focal position in relation to the contact surface.
The invention aims to make available to the user of an fs laser device a routine test that enables a simple examination of the accuracy of the positioning of the focus, in particular in the direction of propagation of the laser radiation (hereinafter called the z-direction). The test and its result are preferentially to be capable of being documented in straightforward manner.
For the purpose of examining the spatial location and orientation of the contact surface of a contact element, designed as an applanation plate, of a laser device, US 2006/0114469 A1 proposes moving the radiation focus along predetermined circular paths and registering, with a photodetector, plasma sparks which arise if the focus at the edge of the applanation plate impinges on the latter.